This invention relates to guiding apparatus for reflecting telescopes used for astrophotography. It is more particularly concerned with off-axis guiding apparatus.
In astrophotography reflecting telescopes are widely used. The telescope is focused on the desired object using the eyepiece in its normal position at the end of the main focusing tube and the eyepiece is then removed and replaced by a camera. As the light received from the object is usually feeble, time exposures are necessary and the telescope is normally provided with a clockwork driving mechanism to keep it pointed approximately at the desired object. In addition to this, however, manual adjustment is also necessary to keep the telescope trained precisely on the desired object. This can be done with a separate guide telescope mounted on the main telescope and kept pointed toward a bright star near the desired object.
However, this arrangement involves two sets of optics mechanically connected and the rotation of the telescope by the driving mechanism causes changes in weight distribution which results in alignment shifts detrimental to sharp photo images. Realignment of the guide telescope is required. It is, therefore, desirable to use the same optics for guiding as for photography.
Some telescope optical systems divide the light to produce two images of the same object, although the images may be of differential magnification. In such devices, the intensity of the first image is reduced by the amount of light that goes to make the second image. Apparatus of that type is shown in U.S. Pat. Nos. 991,652, 2,628,529, 2,753,760 and 3,064,526.
Another type of guiding optical system is known as off-axis guiding. In this type some light is diverted from the periphery of the light cone, relatively close to the systems focusing into a separate eyepiece tube. In this way, a second image that is off-axis from the center of the main image is produced without reducing the intensity of the main image's central light. An off-axis arrangement is disclosed on Page 23 of the book "Astrophotography Near City Lights", copyrighted 1977, by Joseph A. Cocozza. In the apparatus there shown the main focusing tube and camera are mounted on a turntable rotatable about the axis of the main fucusing tube and affixed to the wall of the telescope barrel. An auxiliary or guide focusing tube is also mounted on that turntable parallel to the main focusing tube. On the underside of the turntable and extending into the telescope barrel is mounted a pair of reflecting prisms arranged to divert a peripheral portion of the light directed into the main focusing tube sideways and upwardly into the guide focusing tube. That apparatus overcomes the objections to a separate guide telescope. However, the guide focusing tube must be spaced from the main focusing tube a distance sufficient to clear the camera on the main focusing tube, which requires a relatively large diameter turntable and a correspondingly large hole 6 to 8 inches in diameter in the telescope barrel. The apparatus, therefore, is not adaptable to small telescopes, and may not allow the user to orient the frame of the picture to the object as might be desired.